energy level alignment in metal/oxide/semiconductor and organic dye/oxide systems

179 Pages·2008·3.46 MB·English

ENERGY LEVEL ALIGNMENT IN
METAL/OXIDE/SEMICONDUCTOR AND ORGANIC
DYE/OXIDE SYSTEMS
BY ERIC BERSCH
A dissertation submitted to the
Graduate School—New Brunswick
Rutgers, The State University of New Jersey
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
Graduate Program in Physics and Astronomy
Written under the direction of
Dr. Robert A. Bartynski
and approved by
New Brunswick, New Jersey
October, 2008
ABSTRACT OF THE DISSERTATION
Energy Level Alignment in Metal/Oxide/Semiconductor
and Organic Dye/Oxide Systems
by Eric Bersch
Dissertation Director: Dr. Robert A. Bartynski
The alignment between the energy levels of the constituent materials of metal-oxide-
semiconductorfieldeffecttransistors(MOSFET’s)anddyesensitizedsolarcell(DSSC’s)
is a key property that is critical to the functions of these devices. We have measured
the energy level alignment (band offsets) for metal/oxide/semiconductor (MOS) sys-
tems with high-κ gate oxides and metal gates, and for organic dye/oxide systems. The
combination of UV photoemission spectroscopy (UPS) and inverse photoemission spec-
troscopy (IPS) in the same vacuum system was used to measure both the occupied and
unoccupieddensityofstates(DOS),respectively,ofthesematerialssystems. Additional
soft X-ray photoemission spectroscopy (SXPS) measurements were made of both the
valence bands and core levels of the high-κ systems. The combination of the UPS, IPS
and SXPS measurements were used to determine the band offsets between the high-κ
oxides and the Si substrates of thin film oxide/Si samples.
To find the metal-oxide band offsets, thin metal layers were sequentially deposited
on the oxide surfaces, followed by spectroscopic measurements. These measurements,
combined with the measurements from the clean oxide surfaces, were used to find
the metal-oxide band offsets. Metal-oxide band offset values were also calculated by
the Interface Gap State (IGS) model. We compared the experimental metal-oxide
ii
conduction band offset (CBO) values with those calculated using the IGS model, and
found that they tended to agree well for Ru/oxide and Ti/oxide systems, but not as
well for Al/oxide systems. Through core level spectroscopy, we correlated observations
of the composition of the metallic layers with the trends in agreement between the
experimental and IGS CBO values, which led to the conclusion that the IGS model
gives accurate values for the CBO for systems with chemically abrupt interfaces. Core
level spectroscopy of the MOS systems also showed that Al and Ti overlayers reduced
the interfacial SiO layers of HfO /SiO /Si and Hf Si O /SiO /Si systems, while
2 2 2 0.7 0.3 2 2
leaving the composition of high-κ layers essentially unchanged.
We also measured the energy level alignment for 3 organic dye/oxide systems, N3
dye on rutile TiO (110), N3 dye on anatase TiO nanoparticle, and N3 dye on epitaxial
2 2
ZnO(11¯20) film substrates. For the N3/TiO (110) system, we found the the N3 highest
2
occupiedmolecularorbital(HOMO)was0.9eVabovetheTiO valencebandmaximum
2
(VBM) and the N3 lowest unoccupied molecular orbital (LUMO) was 0.5 eV above the
TiO conduction band minimum (CBM). The energy level alignment for the N3/TiO
2 2
nanoparticle system was similar to that for N3/TiO (110). The alignment between
2
the N3 HOMO and oxide VBM for the N3/ZnO systems was found to be similar to
thoseoftheN3/TiO systems,whereasthealignmentbetweentheN3LUMOandoxide
2
CBM alignment was found to differ markedly between the N3/ZnO and the respective
N3/TiO systems. The difference in the LUMO-CBM alignments is attributed to the
2
different interactions between the N3 LUMO and the ZnO and TiO conduction bands.
2
In addition, we measure the energy level alignment for a prototype dye molecule, ison-
icotinic acid (INA), on TiO (110) and ZnO substrates. These measurements showed
2
that the LUMO of INA is similar to that of N3, and that the HOMO of INA is much
different than that of N3, in keeping with expectations based on the compositions and
theoretical electronic structures of these molecules.
iii
Acknowledgements
There are many people I would like to thank for helping me complete this degree. The
first person I want to thank is my advisor, Bob Bartynski. Bob has been an excellent
advisor to me over my graduate career, providing me with challenging projects and
guiding me along the way as I worked on them. I am most grateful to Bob for his
persistence and enthusiasm on a project that had a number of setbacks and delays.
Without his strong leadership, this project would not have had the positive outcome
that it did. I have also learned much from Bob on a personal level. He is generous
and insightful, and I have developed greatly as a scientist and a person with him as my
advisor.
Ialsowanttothankmylabmates,whomadedailylifeinthelabarichandrewarding
experience. I am grateful to Anthony Danese, who helped to recruit me to the group,
and who was always a willing mentor on matters of the lab or graduate student life.
Thanks, also, to Safak Sayan for training me on how to operate the UHV chamber
and perform the IPS measurements, as well as his career advice in later years. I was
fortunate to join the group around the same time as James Lallo and Hua Yao, who
became my good friends and confidantes for years to come. I want to thank Sylvie
Rangan for being a scientific mentor for me, as well as a friend. Our discussions over
the years have had an impact on my ways of thinking. Working with Levan Tskipuri
has been a great pleasure. His unique way of going about his life and work were a
source of endless surprises. I am very glad that Jean Patrick Theisen joined our group
this past year. He contributed greatly to the project that we worked on together, and
his friendship was an unexpected joy in this last year of my graduate career. I would
also like to thank Lyudmila Goncharova for the MEIS measurements and hosting the
marvelousFridayeveningdinnerparties,andDaveHillforbeingsuchagreatcompanion
in the lab for a number of years.
iv
Another highlight of graduate school for me has been the friends I made in my first
two years at Rutgers. Thanks to Jackie and Howard Bondell, Karen and Jeff Spaleta,
Jessica and Aaron Warren, Neelima Sehgal and Rouven Essig, Matthew and Meredith
Francis, and Jeremy Pronchik for being my friends for all of these years. Your support
has been very important to me. I would also like to acknowledge the other professors
with whom I worked at Rutgers. Rick Garfunkel has been a great second advisor to
me, always willing to give scientific advice and able to raise the spirits in the room. I
have been honored to have some interaction with Torgny Gustafsson and Ted Madey,
two first rate scientists and very admirable men. Elio Vescovo played a major role in
the work presented in this thesis by allowing us to use beamline U5 at the NSLS and
guiding our work there with his Italian wit and wisdom. I would also like to thank
Bill, Eric, Ernie and Arvid. Their work in the machine shop was indispensable to the
maintenance and improvement of the UHV system, and it was always enjoyable to pay
them a visit.
My time at Rutgers would not have been nearly as rich had it not been for my
family. Thanks to Uncle Jack, Aunt Pat, Aunt Barbara and Uncle Anthony for letting
me stay at your houses during my first year at Rutgers. The time I spent with you and
Ryan, Molly, Tommy, Anna and Jacob during that year was a very special to me. I
also want to thank my brothers and sister-in-law for remaining close to me during this
time, though they were cheering me on from Wisconsin. Lastly, I would like to thank
my Dad for his love and support. The perpetual encouragements to ”keep up the good
work” had their effect, and I drew strength from them. I am grateful for the love of
learning that you and Mom instilled in me at an early age, and I hope to continue to
make good use of it in the future.
v
Dedication
To my Dad and Mom
vi
Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Dedication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1. Miniaturization of MOSFETs . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. MOSFET Band Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Candidate High-κ Materials . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. MOSFET Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5. Flatband Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.6. Band Alignment Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.7. Alternative Gate Electrode Materials . . . . . . . . . . . . . . . . . . . . 17
1.8. Importance of Band Offsets . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.9. Measurement of Band Offsets . . . . . . . . . . . . . . . . . . . . . . . . 21
1.10.Energy Level Alignment in Dye Sensitized Solar Cells . . . . . . . . . . 23
1.11.Outline of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2. Ultra High Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3. Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4. Inverse Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . 37
vii
2.5. Auger Electron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 42
2.6. Low Energy Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . 45
2.7. X-ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 47
3. Oxide-Semiconductor Band Offsets . . . . . . . . . . . . . . . . . . . . . 49
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.2. Growth of High-κ films on Si . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3. Interfacial SiO Oxide Layer . . . . . . . . . . . . . . . . . . . . . . . . . 51
2
3.4. Comment on Sample Preparation . . . . . . . . . . . . . . . . . . . . . . 52
3.5. Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.6. Preliminary Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.7. HfO -Si Band Offsets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
2
3.8. Predicted HfO -Si Band Offsets . . . . . . . . . . . . . . . . . . . . . . . 62
2
3.9. Hf Si O -Si Band Offsets . . . . . . . . . . . . . . . . . . . . . . . . . 64
0.7 0.3 2
3.10.ZrO -Si Band Offsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2
3.11.Al O -Si and SiO -Si Band Offsets . . . . . . . . . . . . . . . . . . . . . 67
2 3 2
3.12.Importance of Sample Preparation; Specific Cases . . . . . . . . . . . . 70
3.13.Si Band Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.14.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4. Metal Oxide Band Offsets . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.2. Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.3. Metal-Oxide Band Offsets for Ru/HfO /Si . . . . . . . . . . . . . . . . . 79
2
4.4. Metal-Oxide Band Offsets for Al/HfO /Si and Ti/HfO /Si . . . . . . . 83
2 2
4.5. Energy shifts for Ru/, Al/, and Ti/Hf Si O /Si . . . . . . . . . . . . 85
0.7 0.3 2
4.6. Energy shifts for Ru/, Al/, and Ti/SiO /Si . . . . . . . . . . . . . . . . 87
2
4.7. Energy Shifts for Ru/, Al/ and Ti/Al O . . . . . . . . . . . . . . . . . 90
2 3
4.8. Theoretical Metal-Oxide Band Offsets . . . . . . . . . . . . . . . . . . . 93
4.9. Discussion of Experimental and Theoretical CBO Values . . . . . . . . . 98
viii
4.10.Effect of Metal Overlayers on the Composition of Oxide and Interfacial
SiO layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
2
4.11.Discussion of IL Reduction in Al/HfO /Si and Ti/HfO /Si . . . . . . . 109
2 2
4.12.Metal-Oxide Band Offset Results from the Literature . . . . . . . . . . . 110
4.13.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5. Energy Level Alignment of Organic Dye/Oxide Systems . . . . . . . 115
5.1. Introduction: Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.2. Introduction: Dye Sensitized Solar Cells . . . . . . . . . . . . . . . . . . 116
5.3. Properties of the TiO Film and Dye Molecule . . . . . . . . . . . . . . 118
2
5.4. Improvement of DSSCs: Dye-TiO Interface . . . . . . . . . . . . . . . . 120
2
5.5. Measurement of Energy Level Alignment . . . . . . . . . . . . . . . . . . 123
5.6. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.7. STM Imaging of N3/TiO (110) Surface Preparation . . . . . . . . . . . 130
2
5.8. Energy Level Alignment: N3/rutile TiO (110) . . . . . . . . . . . . . . . 131
2
5.9. Energy Level Alignment: N3/TiO nanoparticle film . . . . . . . . . . . 136
2
5.10.Energy Level Alignment: N3/ZnO (11¯20) epitaxial film . . . . . . . . . 139
2
5.11.Energy Level Alignment: INA/TiO and INA/ZnO(11¯20) . . . . . . . . 142
2
5.12.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
7. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
7.1. List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Vita . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
ix
List of Tables
1.1. International Technology Roadmap for Semiconductors (ITRS) values
(2006)showingtheoxidethicknessatwhichgateleakagecurrentbecomes
unacceptable. [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Dielectric constants and band gaps for candidate high-κ dielectrics. . . . 8
3.1. Measured values for HfO band gap and HfO -Si VBO and CBO. . . . . 63
2 2
3.2. Measured values for ZrO band gap and ZrO -Si VBO and CBO. . . . . 66
2 2
3.3. Measured values for Al O band gap and Al O -Si VBO and CBO. . . . 69
2 3 2 3
3.4. Measured values for SiO band gap and SiO -Si VBO and CBO. . . . . 70
2 2
4.1. The experimental values for the metal-oxide VBO and CBO are deter-
mined by adding the VBM and CBM of the oxide before metallization
and the energy shift (E ) upon metallization (and taking the absolute
s
value for the VBO). Metal-oxide band offsets values determined with the
IGS model are also shown for comparison. . . . . . . . . . . . . . . . . . 97
4.2. Experimental and theoretical values for the metal-oxide CBO. . . . . . . 111
7.1. List of acronyms used in this thesis. . . . . . . . . . . . . . . . . . . . . 152
x